1. Technical Field
This invention relates to liquid level sensing and, more particularly, to such sensors which are used for detecting high water levels in the bilge areas of boats and for controlling bilge pumps.
2. Description of the Prior Art
High water levels in the bilge area of a boat can be detected by electronic or mechanical sensor/switches. The bilge of a boat, however, is a harsh environment for a bilge pump trigger switch, especially in a salt water environment. This condition often causes both mechanical and electrical water level sensors to fail due to debris within the bilge, fowling and degradation from oily water, and corrosion from salty water. Additionally, electronic sensor/switches usually require a constant source of power to maintain sensing capability, presenting battery depletion problems.
Automatic detection of water in the bottom of boats is most commonly performed with a mechanical float switch such as the one disclosed in U.S. Pat. No. 4,697,535. This type of switch typically uses mechanical contacts that close when water levels are high enough to lift the float. Failure can occur when debris and corrosion jam the float, or when corrosion prevents the electrical contacts from closing. Proper mounting of a float switch is critical since placement requires enough room to allow free movement. In addition, sloshing bilge water frequently triggers the pump causing unnecessary pump cycling. Frequent cycling of the pump discharges the battery prematurely and causes excessive wear of pump and motor components.
Other bilge pump switches have been developed which are triggered in response to air pressure. This type of switch utilizes a tube closed at one end mounted vertically with the closed end upward. Disposed within the tube is a diaphragm and microswitch mounted thereon. The tube is sealed at a lower end with an inverted moveable cup. When bilge water reaches the lower end of the tube and rises still further, the water within the inverted cup is urged upward within the tube and increases internal tube pressure which displaces the diaphragm, thus closing the switch and triggering the bilge pump. Disadvantages of this system include leakage of water into the sealed portion of the air tube due to deterioration of the diaphragm or tube, and deterioration of plastic parts in the salty, oily water of the marine environment. For example, the inverted cup degrades from the presence oil in the water, often requiring replacement. Also, the microswitch mechanism becomes unreliable in such an environment.
Recently, electronic sensor/switches have been developed for water level sensing. U.S. Pat. Nos. 4,897,822 (Korten et al.), and 4,881,873 (Smith et al.) disclose fully electronic liquid level sensors, automatic activation/deactivation of a bilge pump, and time delay circuitry to prevent false activation/deactivation of the pump. The bilge level is detected by measuring a change in media density proximal to acoustic sensors. In these designs, one acoustic transducer transmits acoustic waves to a second receiving transducer. Since bilge water has a greater density than air, a greater amount of acoustic energy is transmitted through bilge water than air. Thus, when bilge water comes into proximity with the transducers, the pump is activated in response to a predetermined increase in transmitted acoustic energy.
A second type of electronic switch is a conductive type switch. Korten et al discuss prior art "probes which utilize the conductivity of the fluid being sensed in contact with the probes creating an electrical path to activate the bilge pump." (See Col. 1, Lines 33-41). These switches typically contain integrated circuits and demand constant power to sense high water levels. They use two electrical contacts one of which is usually submersed in the water; therefore cleaning is recommended. Conductive switches utilize sealed electronics located in the bilge. Accordingly, disadvantages include the need for adequate mounting space, and susceptibility of circuitry to the bilge environment.
Still another type of liquid sensing switch utilizes a single probe. An example is the Bilge Tender liquid sensing switch (referred to as the "Bilge Tender switch", which has previously been manufactured and sold by AIM Technologies of Cape Coral, Fla. This device uses the ship's hull surface as a path to ground to complete a circuit when water touches the probe. The Bilge Tender switch utilizes a remotely located single wire probe for sensing immersion in low conductivity liquids or in liquids where no hardwired ground is provided. The Bilge Tender switch also automatically activates/deactivates a bilge pump, incorporates time delay circuitry to prevent false triggering, and is fully electronic.
Referring to FIG. 1, a block diagram of a single probe liquid sensing switch of the prior art Bilge Tender switch is shown. Probe 5 is located within a bilge portion of a boat hull. Probe input amplifier 10 provides the necessary sensitivity and turn-on delay for single probe operation of the liquid sensitivity switch. Since the output voltage (depicted at reference numeral 15) of probe input amplifier 10 increases over time, the output is inverted by inverter 20 for input to integrated monostable stage 30. Thus, inverter 20 inverts the output of probe input amplifier 10 to provide a decreasing signal to trigger integrated monostable stage 30 at its threshold voltage. When integrated monostable stage 30 is triggered, output 35 triggers output transistor driver 40. Output 45 from output transistor driver 40 activates high current PNP load driver 50 which provides the necessary current to drive pump 60, via line 55.
Output transistor driver 40 is necessary in this prior art design since output 35 from integrated monostable stage 30 is not high enough to drive the output transistor pair within the high current PNP load driver 50 directly.
FIG. 2 illustrates the prior art bilge switch of FIG. 1 in greater detail. Here, the negative terminal of the battery is in contact with the bilge water. When probe 5 becomes immersed in bilge water from the accumulation of water in the bilge area of the boat, capacitor C1 begins to charge. This charging period T1 represents the initial time delay between the time at which probe 50 contacts the water and the time at which U1 is activated (due to transistors Q1 and Q2). If probe 60 emerges from the water before time T1 has elapsed, capacitor C1 discharges through resistors R1 and R2. The time delay T1 is a function of bilge conductivity (which dictates the current flowing though probe 5), and the values of C1, R1, R2, and R3. Given a desired time delay T1, the respective values of the components are appropriately chosen.
When the voltage at node A drops a predetermined value below 12 V (typically about 0.4 V), output 15 from transistor Q1 increases to a predetermined value from 0 (typically 8 V). Since output 15 from transistor Q1 increases after it is triggered, output 15 must be inverted to properly trigger the 555 integrated circuit U1. Thus, transistor Q2 inverts output 15 from transistor Q1 to create decreasing output 25 at node C. When the voltage at node C drops near the saturation voltage of transistor Q2, the 555 integrated circuit U1 triggers and remains activated for a run time T2, as determined by the values of resistor R8 and capacitor C2. If water coming into the bilge is not pumped below the level of probe 5 during the run time T2, the 555 integrated circuit U1 immediately triggers again. On the other hand, if the probe 5 is out of water for a period greater than T1, the 555 integrated circuit U1 finishes its cycle of time T2 and deactivates. During activation time T2, output 35 from U1 is not sufficient to drive Q4 directly. Thus, output transistor Q3 provides the necessary current for output transistor pair Q4. Output 45 from transistor Q3 drives output Darlington PNP transistor pair Q4, activating pump 60 via output 55.
The Bilge Tender single probe device as described above has several disadvantages. Probe input sensitivity in this prior art device is approximately 10,000 ohms. This relatively low sensitivity makes this device particularly vulnerable to probe corrosion or fowling, which enhances the likelihood of failure due to diminishing probe sensitivity. Additionally, the 555 timing integrated circuit U1, which receives output from the sensitivity transistors Q1 and Q2, creates a minimum current draw of 10-20 milliamperes, which can drain the battery of a boat over a relatively short period of time. Also, the overall drive requirement of the Bilge Tender switch is rather high at about 1 ampere. Furthermore, the transistor pair Q4 generates excessive heat and requires a heat sink. Finally, Q4 must be thoroughly insulated from ground since it is a large Darlington PNP driver, which greatly increases the cost of the system.
Table I below illustrates the values for all resistances in FIG. 2, as well as the types of transistors. The integrated circuit type is listed also.
TABLE I ______________________________________ Component Value/Part Number ______________________________________ R1 47 KOhm R2 4.7 KOhm R3 10 KOhm R4 27 KOhm R5 27 KOhm R6 4.7 KOhm R8 470 KOhm R9 10 KOhm R10 2.2 KOhm R11 390 KOhm, 1 Watt R12 4.7 KOhm R13 10 KOhm C1 100 Microfarads Q1 2N2906 Q2 2N2222 Q3 2N6388 Q4 2N6286 U1 555 Timing Integrated Circuit Pump Any 12 v pump ______________________________________
Finally, each of the aforementioned prior art electronic switches have a variety of problems. They utilize probes with low sensitivities, expose too many electronic components to the bilge water, require a minimum current draw that is unacceptably high, etc.